X-ray microscope with zone plates

ABSTRACT

Light-intensive zone plates (4) are disclosed which are useful as condensers and X-ray objectives for high resolution X-ray microscopes. They have high refraction effectiveness in a high refraction order thanks to a high aspect ratio (H/P) and a suitably adjusted line-slot ratio (P 1  /P 2 ) lower than 1. Additional improvements may be obtained by zones (6, 7) inclined relative to the optical axis (3). The zone plates (4) may also be operated in Bragg reflection. They thus provide efficient optics with a high numeric aperture and make X-ray microscopes with 10 nm resolution possible. The zone plates (4) may have a relatively coarse structure, and thus they are easy to produce in a relatively short time. The zone plates (4) with high numerical aperture may be used in a particularly advantageous manner as small condensers in laboratory X-ray microscopes, as they can capture light from a microplasma X-ray radiation source in a particularly wide solid angle and focus it on an object.

Th e invention relates to an X-ray microscope with zone plates for acondenser-monochromator and for a microscope objective.

In X-ray microscopy, substantial progress has been made over recentyears in the wavelength region of approximately 0.2-5 nm. X-raymicroscopes have been developed which are being operated on brilliantX-ray sources. Electron storage rings emit strongly focused X-rayradiation. Also included in the development are compact X-ray sourceswhich are intended for the use of X-ray microscopes in the laboratory.Such X-ray sources can consist of hot microplasmas (typical diameter ofthe radiating region: 10-50 μm) which are generated with the aid ofpulsed laser beams. They radiate their X-ray light in all spatialdirections.

At present, only microscope zone plates are used as highly resolvingobjectives in X-ray microscopes. Microscope zone plates are rotationalsymmetrical circular transmission gratings with grating constants whichdecrease outward, and typically have diameters of up to 0. 1 mm and afew hundred zones. The numerical aperture of a zone plate is determinedvery generally by the diffraction angle at which the outer, and thusfinest zones diffract vertically incident X-ray beams. The achievablespatial resolution of a zone plate is determined by its numericalaperture. Over recent years, it has been possible for the numericalaperture of the X-ray objectives used to be substantially increased,with the result that their resolution has improved. This trend to higherresolution will continue.

It is known from the theory of microscopy that the numerical aperture ofthe illuminating condenser of a transmitted-light microscope shouldalways be approximately matched to the numerical aperture of themicroscope objective, in order also to obtain an incoherent objectillumination from incoherently radiating light sources, and thus toobtain a virtually linear relationship between object intensity andimage intensity. If the aperture of the condenser, by contrast, is lessthan that of the microscope objective, a partially coherent image ispresent, and the linear transformation between object intensity andimage intensity is lost for the important high spatial frequencies,which determine the resolution of the microscope.

A condenser of high light-gathering power must be used for it to bepossible to use the X-ray sources in a simple and matched [sic] way forbright-field microscopy, phase contrast microscopy and, in particular,dark-field microscopy. Normally, use is also made as condensers ofdiffracting optical systems, for example zone plates, since these may beused to render the X-ray radiation monochromatic at the same time. Suchzone plates are to have a diffraction efficiency that is as high aspossible, in order to focus as much of the captured radiation aspossible onto the object.

Such "condenser zone plates" are normally used at the first diffractionorder, at which all condenser zone plates implemented to date have theirhighest diffraction efficiency. It is difficult in this case to achievethe previously required matching of the numerical aperture of thecondenser zone plates to that of the microscope zone plate (X-rayobjective). In order to realize the matching, the condenser zone platemust have the same fine zones on the outside as does the microscope zoneplate itself. The microscope zone plates built with the highestlight-gathering power meanwhile have zone widths of only 19 nm(corresponding to a 38 nm period of the zone structures). Zone plateswith such fine zone structures can so far be produced only using methodsof electron beam lithography, in which the zones are producedsuccessively. Holographic methods, which produce the pattern of a zoneplate in one step in a "parallel" fashion and thus in a short time areruled out, since a suitably shortwave UV holography does not exist.Consequently, it would also be possible to produce condenser zone plateswith matched numerical apertures only using methods of electron beamlithography, and this must be described as a serial, and thus slowmethod. However, such condenser zone plates have not yet been producedto date.

Condenser-monochromator arrangements of even higher light-gatheringpower and having an annular hollow conical aperture are required fordark-field X-ray microscopy, if an absorbing ring, which is to beadjusted very precisely, is not placed in the rear focal plane of themicroscope objective. The periods of the zone structures of suitablecondenser zone plates would, in turn, need to be less than 38 nm.

A condenser-monochromator arrangement which as far as possible deliversall the X-ray light made available by the beam tube into an annularhollow conical aperture of large aperture angle relative to the objectis advantageous for phase-contrast X-ray microscopy.

Object illumination of hollow conical shape is generally required forX-ray microscopes which use zone plates as X-ray objectives. Otherwise,the radiation from the zero and the first diffraction orders of thecondenser zone plate would also overlap the image at its center. Thereason for this is that the overwhelming proportion of the radiationwhich falls onto the object in a fashion parallel or virtually parallelto the optical axis penetrates said object and the following microscopezone plate (the X-ray objective) without being diffracted and is seen asa general diffuse background in the direction straight ahead, that is tosay in the center of the image field. For this reason, all transmittingX-ray microscopes use annular condensers, and the useful region, notdiffusely overexposed region, of the image field becomes larger thelarger the inner, radiation-free solid angle region of the condenser.

In order to improve the resolution of the X-ray microscopes to 10 nm,work is presently being carried out on developing microscope zone plateswhich have a minimum zone width of only approximately 10 nm. Thisincreases the apertures of the microscope zone plates and, consequently,the required numerical apertures of the condensers, in order to ensurean incoherent object illumination. The already mentioned difficultiesare thereby compounded further.

Such highly resolving microscope zone plates would need to have zoneswith a structural width of approximately 10 nm. However, so far nosuccess has been achieved nor explanation given as to whether suchexposed zone structures carried by a backing foil, which generallyconsists of a metal such as germanium or nickel, can still be producedwith the aid of electron beam lithography and transmitted into metal. Ithas also not been shown for sputtered-sliced zone plates that it ispossible to use the sputter method for such small structural widths toproduce sufficiently stable zone rings which are not disturbed bymaterial diffusion and can finally be further processed into a zoneplate by means of thinning methods, it being the case in particular,that the zones should preferably be capable of being etched out ofmaterial of low scattering power, thus producing the profile of alaminar structure.

It is generally known from diffraction theory in optics that, withhigher diffraction orders, it is possible in principle to achieve higherapertures, and thus a spatial resolution which is higher by the factorof the diffraction order m. If the finest structural width is 30 nm, forexample, something which is simple to produce, a resolution of 10 nmwould theoretically be possible in the third diffraction order. However,in this case it would also be necessary to reach a diffractionefficiency which far exceeds that of the other diffraction orders.

The diffraction efficiency of zone plates as X-ray optical systems hasso far been calculated within the framework of an approximation ingeometrical optics. In this case, it has been assumed that the aspectratio of the zone structures, that is to say the ratio of the zoneheight to the length of the zone period is distinctly smaller than 10:1.According to this approach, it is impossible in principle to expect ahigh diffraction efficiency at high diffraction orders. On the contrary,the maximum possible diffraction efficiency scale with 1/m² for thediffraction orders m=1,3,5 . . . , with the result that only a fewpercent is possible according to this model. The diffraction efficiencyis correspondingly lowered for the third diffraction order by the factor˜1/m² =(1/3)² =1/9, at least, with the result that light is scarcelyavailable any more at the higher diffraction order. The contrast of animage is therefore strongly attenuated by the radiation of theremaining, much more efficient diffraction orders. In practice, it hastherefore not been possible so far to use zone plates at higherdiffraction orders.

Again, it is known from the theory of coupled waves, applied to zoneplates, that when they have an aspect ratio >1 zone structures canassume a particularly high diffraction efficiency only at their firstorder (up to approximately 50% for materials which are suitable forX-ray optics and realistic, that is to say can be technicallyprocessed). The precondition for this is that zone structures extendalong the surfaces of constant phase, which can be constructed for anobject point on the optical axis and for the associated image point. Ifsaid surfaces extend parallel to and concentrically with the opticalaxis, the zone structures act like the lattice planes of a crystal whichis used with Bragg reflection and which therefore fulfills the Braggcondition. In very general terms, Bragg reflection occurs when the zonestructures are inclined such that they extend parallel to the anglebisector ("Bragg angle") of the incident and diffracted beam directions.The talk will therefore be of "zone plates with Bragg reflection" forsuch a case in what follows.

Furthermore, a theoretical description based on the wave equation(theory of coupled wave) has been used to calculate the diffractionefficiency, in order to obtain more accurate data for the efficienciesof first order or higher aspect ratios, as well. A Fourierrepresentation with a line/slot ratio of 1:1 has been used in the waveequation to describe the grating structures of the zone plate. Theline/slot ratio specifies the ratio of the structural widths of the zonematerial which strongly scatters the X-ray radiation, and that whichweakly scatters it. The system of differential equations resultingtherefrom has been numerically integrated, something which required manyhours for one calculation even on a high-speed computer (for example IBMRS-6000), even in the case of layer thicknesses of less than 1 μm.However, in this connection it is only the first order which has beenconsidered as imaging order. The theoretical results for the diffractionefficiencies agreed to a very good approximation with the approach ofgeometrical optics in the case of aspect ratios up to a maximum ofapproximately 5:1. Only at higher aspect ratios and with inclination ofthe zone structures was it possible for higher efficiencies to becalculated in accordance with the model in geometrical optics. It has sofar seemed to be impossible, both in accordance with the model ingeometrical optics and from the theory of coupled waves, to obtain highdiffraction efficiencies for higher diffraction orders (m=2,3, . . . )as well. Experimental results have also not indicated this in any way.

It is the object of the invention to represent an X-ray microscope witha resolution of at least 10 nm, and to specify for this zone plateswhich can be operated at higher diffraction orders, the aim being toachieve at the higher diffraction orders diffraction efficiencies atleast at a level such as is exhibited by the known zone plates operatedat the first diffraction order, and whose zone structures can bedistinctly coarser than 10 nm, and which are suitable for use incondenser-monochromator arrangements and as microscope objective.

This object is achieved according to the invention by means of thefeatures specified in the characterizing part of claim 1. Advantageousembodiments and developments of the invention follow from the subclaims.

A resolution of 10 nm can be achieved if the specified zone plates areused in an X-ray microscope as a condenser-monochromator and as amicroscope objective. The diffraction efficiency of said zone platesreaches its maximum at a higher diffraction order by means of a suitablyset line/slot ratio of less than 1:1 and a high aspect ratio. EfficientX-ray optical systems with the necessary high numerical aperture arethereby available. In addition, they render X-ray microscopes with a 10nm resolution possible, without the need to use the extremely small zonestructures, technically exceptionally difficult to produce, which wouldbe necessary for zone plates of the same resolution in the case of theuse of the first diffraction order. At the same time, a diffractionefficiency which it has so far been possible to achieve only at thefirst diffraction order is achieved at this higher diffraction order.Such zone plates with a high diffraction efficiency and a high numericalaperture can be used in laboratory X-ray microscopes with particularadvantage as small condensers which capture light from a microplasmaX-ray radiation source from a particularly high solid angle, and focusit on the object.

The way of achieving the object set could only be via a comprehensiveanalytical description of the diffraction behavior of zone plates whichprovides an overview of all diffraction orders, different line/slotratios and much larger zone heights. Because of the enormous rise incomputation time required, this object was ruled out with the numericaliterative methods of calculation to date.

There were two problems to overcome in this case. Firstly, it wasnecessary to find another mathematical method for distinctly shorteningthe computation time, in order to be able to calculate even large aspectratios sufficiently quickly. On the other hand, it was necessary for theline/slot ratio to be incorporated into the wave equation as a furtherparameter, and said ratio distinctly complicates the Fourierrepresentation of the grating, and thus the wave equation. The resultwas a system of differential equations which was solved as acomplex-value eigenvalue problem, complex-value matrices occurring up toa dimension of 100×100 elements. This method of solution reduced thecomputation times by a factor of approximately 1000. The efficiency ofany diffraction order can be represented as a function of the zoneheight. It has been shown that the diffraction efficiency at high orders(for example m=6) can be drastically raised if the line/slot ratio isselected to be smaller than 1:1, the zones have a high aspect ratio and,in addition, the zone structures are arranged in a fashion similar tosmall mirrors with Bragg reflection.

This had not been known to date, and is to be understood only by meansof a comparison, not drawn until now, relating to the mode of operationof multilayers. In practice, this effect can be utilized for the purposeof realizing high diffraction efficiencies and high apertures in X-rayoptical systems, without at the same time being dependent on theproduction of extremely narrow zone structures, as would be necessaryfor operation at the first diffraction order.

It has emerged that a zone plate with a high aspect ratio (typicalvalue: greater than 10) has a comparatively high diffraction efficiencyat one of its high diffraction orders, like a zone plate with a highaspect ratio used at the first diffraction order, if said line/slotratio is distinctly smaller than one. Since such a zone plate is used ata high diffraction order, it has a greatly increased aperture--comparedwith applications at the first diffraction order. For example, a zoneplate with a high aspect ratio (approximately 20) and a low line/slotratio (approximately 0.25) can have a diffraction efficiency of up to45% if it is used at the sixth diffraction order and with Braggreflection at a wavelength of 2.4 nm. Materials suitable for X-rayoptics and capable of being processed technically are used for thispurpose. It holds in very general terms that the parameters of the zoneplate such as, for example, materials, aspect ratio and line/slot ratiocan be optimized for the higher diffraction order respectively desired.

Given the use of a higher diffraction order and Bragg reflection--it isan advantage of zone plates with a large aspect ratio and smallline/slot ratio that in the case of the same numerical aperture a zoneplate used at a high diffraction order requires only relatively coarsezone structures by comparison with a zone plate of the same numericalaperture used at the first diffraction order. For the above example ofan X-ray microscope with a resolution of 10 nm, the result for thefinest zone structure to be produced is a width of approximately 30 nmwith a period of 120 nm, if the zone plate is to be operated at thesixth diffraction order. Such structural widths can be effectivelyproduced at the present time using means of electron beam lithography.In addition, zones 6 times smaller are to be written, and this proceedssubstantially more quickly. For a zone plate condenser written byelectron beam, this means that the write times are drastically reduced.

A zone plate for Bragg reflection can be reduced using known vapordeposition techniques, for example according to the known method forproducing so-called sputtered-sliced zone plates by sputter coating of apolished wire rotating in a vacuum, the materials suitable for X-rayoptics being applied alternately. The wire with the materials applied issubsequently embedded in a substrate and cut into disks at right-anglesto its axis. This produces zone plates whose inner region is absorbing,that is to say inactive in terms of X-ray optics, and this is desiredfor the condenser on the grounds set forth in the introduction.

Instead of a wire, it is possible to use an optically polished metal orglass ball as an alternative method for producing a zone plate. Theball--which is rotating--is coated in a vacuum with a multilayer systemand subsequently thinned on its circumference down to a ball zone with awidth of a few μm near its equator. If the thinned ball zone is notsituated exactly on the equator of the ball, the remaining layersequence is inclined. If the inclination is half as large as therequired beam deflection and corresponds to the above-named anglebisector, the layer sequence is at the Bragg angle. The layer sequenceacts like a multiple mirror, with the result that a maximum is achievedin the diffraction efficiency.

Diagrammatically represented exemplary embodiments of the invention areexplained below in more detail with the aid of the drawing, in which:

FIG. 1 shows a zone plate according to the invention,

FIG. 2 shows an X-ray microscope with condenser and microscope zoneplate s, both of which are operated with Bragg reflection,

FIG. 3 shows an X-ray microscope with condenser and microscope zoneplates, both of which have inclined zones and are operated with Braggreflection, and

FIG. 4 shows an X-ray microscope having a focussing device withfocussing ring and a downstream annular zone plate and a microscope zoneplate.

An exemplary embodiment of a zone plate 4 according to the invention isrepresented diagrammatically in cross section in FIG. 1. The diffractingproperties of the zone plate 4 are determined by the line/slot ratio P₁/P₂, the aspect ratio H/P and by the inclination of the zones 6,7 withrespect to the optical axis 3. Of course, in this case the materials ofthe zones 6,7 which are active in terms of X-ray optics, also play arole. The line/slot ratio P₁ /P₂ specifies the ratio of the structuralwidth of the material of the zones 6, which strongly scatters theincident X-ray radiation 1, to the structural width of the material ofthe zones 7 which is weakly scattering. The line/slot ratio P₁ /P₂ isconstant over the entire zone plate 4. The aspect ratio specifies theratio of the zone height H to the length P of the zone period, andincreases in this exemplary embodiment, starting from the optical axis 3toward the edge of the zone plate 4.

According to the invention, a high diffraction efficiency is achieved ata higher diffraction order when the line/slot ratio P₁ /P₂ is smallerthan 1, as is represented, for example, with 0.5 in a fashion true toscale in FIG. 1, and when a large aspect ratio such as, for example,greater than 10 is realized, which is not, however, represented true toscale in FIG. 1.

A further increase in the diffraction efficiency at a higher diffractionorder can be achieved for specific applications with zones 6,7, whichare inclined with respect to the optical axis 3. The exemplaryembodiment in accordance with FIG. 1 shows zones 6,7 which extend nearthe optical axis 3 and parallel to said axis. With increasing spacing ofthe zones, 6,7 from the optical axis 3, there is also an increase in theinclination of zones 6,7 with respect to the optical axis 3. A furtherimprovement can be achieved when the zone plate 4 with its zones 6,7 areused with Bragg reflection.

The X-ray radiation 1 incident on the zone plate 4 is diffracted withdifferent intensities at different diffraction orders. FIG. 1 shows thepropagation directions for the diffraction of zero order 8, first order9a, second order 9b and third order 9c. The diffraction angle increaseswith the higher diffraction orders. It is therefore possible to achievea high aperture, and thus a high resolving power of the X-ray microscopewith a high diffraction order when the zone plate 4 is used as condenserand/or as objective in an X-ray microscope. In this case, coarsestructures, which can advantageously be produced easily and in arelatively short time, suffice as zones 6,7 of the zone plate 4.

FIGS. 2-4 show diagrams of zone plates 4 in arrangements as condensersand microscope zone plates for X-ray microscopes with particularly highresolution, which are operated with various radiation sources.

FIG. 2 represents the optical system of an X-ray microscope in which anisotropically radiating microplasma X-ray source 17 serves as radiationsource. A suitable condenser in this case is an annular zone plate 14with non-inclined zones 6,7, which are advantageously operated withBragg reflection. The zone plate 14 focuses the X-ray radiation 1 of themicroplasma X-ray source 17 via a hollow cone of radiation 10 at thefocus 13 on the optical axis 3. The object thereby illuminated islocated there. Also arranged at said point is a monochromator pinholediaphragm 11, which masks out the undesired diffraction orders andwavelengths of the X-ray light of a further beam path. The zone plate 14thereby cooperates with the monochromator pinhole diaphragm 11 as acondenser-monochromator which is used generally for illuminating objectsin X-ray microscopes.

A microscope zone plate 12 with incline d zones 6,7 and with Braggreflection serves as X-ray objective. Said plate generates an image ofthe object in the image plane 18.

As already mentioned in the introduction, in order to eliminate thenon-diffracted X-ray radiation as diffused background the zone plate 14and the microscope zone plate 12 have a central zone plate region 19which absorbs the X-ray radiation.

Represented in FIG. 3 is the optical system of an X-ray microscope whichmakes use as optical elements of a condenser zone plate 15 with Braggreflection and inclined zones, and of a microscope zone plate 12 withBragg reflection and inclined zones 6,7. The X-ray radiation 1, incidentin a virtually parallel fashion, of an undulator or a deflecting magneton an electron storage ring is focused at a high aperture angle and withhigh diffraction efficiency in an object in the plane of themonochromator pinhole diaphragm 11. In order to effect Bragg reflectionin this application, the zones 6,7 of the condenser zone plate 15 mustbe inclined. The central zone plate region 20 absorbing the X-rayradiation comprises a spherical carrier.

Represented in FIG. 4 is an X-ray microscope having a focussing device21 with focussing ring and an annular zone plate 16, downstream in thebeam path, with Bragg reflection and inclined zones 6,7. Together with amonochromator pinhole diaphragm 11, the focussing device 21 and the zoneplate 16 form a condenser-monochromator. The focussing device 21 withfocussing ring focuses the incident X-ray radiation 1, focussed inparallel, of an undulator or a deflecting magnet of an electron storagering in the form of a ring. The zone plate 16 is arranged near thefocussing ring of the focussing device 21. The zones 6,7 of the zoneplate 16 are modified such that they generate a punctiform focus 13 onthe optical axis 3 by diffraction from the focussing ring of thefocussing device 21. It is advantageous in this arrangement that thezone plate 16 does not need to have a large area, since it can belocated near the focussing ring of the focussing device 21. Only a fewstructures therefore need to be produced on the zone plate 16. Thelight-collecting area is determined solely by the focussing device 21.It has only coarse zone structures, and can therefore be effectivelyproduced using methods of electron beam lithography. This arrangementcan be applied with particular advantage for well collimated X-rayradiation 1, for example from an undulator.

A microscope zone plate 12 with Bragg reflection and inclined zones 6,7serves as X-ray objective in the case of this condenser-monochromatorarrangement as well.

List of-Reference Symbols

1 Incident x-ray radiation

3 Optical axis

4 Zone plate

6 Zone with material of high scattering power

7 Zone with material of low scattering power

8 Beam of zero diffraction order

9a Beam of first diffraction order

9b Beam of second diffraction order

9c Beam of third diffraction order

10 Illuminating hollow cone of radiation

11 Monochromator pinhole diaphragm in the object plane

12 Microscope zone plate with high aspect ratio and inclined zones

13 Focus in the object plane

14 Annular zone plate with high aspect ratio and non-inclined zones

15 Annular zone plate with high aspect ratio and inclined zones

16 Annular zone plate with Bragg reflection with high aspect ratio andinclined zones

17 Microplasma x-ray source

18 Image plane

19 Central, absorbing zone plate region

20 Central, absorbing zone plate region composed of spherical carriers

21 Focussing device with focussing ring

H Zone height

P Period of the zones

P₁ /P₂ line/slot ratio

I claim:
 1. X-ray microscope with zone plates for acondenser-monochromator and for a microscope objective, characterized inthat at least one zone plate is provided which is arranged on theoptical axis of the X-ray microscope and has a high aspect ratio (H/P)and a line/slot ratio (P₁ /P₂) smaller than
 1. 2. X-ray microscopeaccording to claim 1, characterized in that the aspect ratio (H/P)increases toward the edge of the zone plate.
 3. X-ray microscopeaccording to claim 1, characterized in that the central region of thezone plate is absorbent for X-ray radiation.
 4. X-ray microscopeaccording claim 1, characterized in that the zones of the zone plate arealigned parallel or inclined to the optical axis.
 5. X-ray microscopeaccording to claim 1, characterized in that in the region near theoptical axis (3), the zones (6, 7) of the zone plate (4, 12, 14, 15, 16)are aligned parallel to said axis, and are increasingly inclined withrespect to the optical axis (3) toward the edge of the zone plate (4,12, 14, 15, 16).
 6. X-ray microscope according to claim 1, characterizedin that a zone plate operating with Bragg reflection is provided. 7.X-ray microscope according to claim 1, characterized in that the zoneplate is annularly constructed for a condenser-monochromator, and amonochromator pinhole diaphragm is arranged at its focus.
 8. X-raymicroscope according to claim 1, characterized in that a focussingdevice with a focussing ring, and a zone plate of annular constructiondownstream in the beam path are provided for a condenser-monochromator,a monochromator pinhole diaphragm being arranged at the focus of thezone plate.
 9. X-ray microscope according to claim 1, characterized inthat the zone plate is used as a microscope objective.
 10. X-raymicroscope according to claim 1, characterized in that zone plates areprovided whose zones are applied to a wire or a polished ball.
 11. Azone plate with diffraction structure for X-ray radiation, the zoneplate being characterized by a high aspect ratio (H/P) and a line/slotratio (P₁ /P₂) smaller than
 1. 12. A zone plate as claimed in claim 11,wherein the aspect ration (H/P) increases toward an edge of the zoneplate.
 13. A zone plate as claimed in claim 11, wherein the centralregion of the zone plate is absorbent for X-ray radiation.
 14. A zoneplate as claimed in claim 11, wherein the zone plate is a zone plateoperating with Bragg reflection.
 15. A zone plate as claimed in claim11, wherein zones of the zone plate are applied to a wire or a polishedball.
 16. A zone plate as claimed in claim 11, wherein the zone plate isarranged in an X-ray microscope perpendicular to an X-ray microscopeoptical axis.
 17. A zone plate as claimed in claim 11, wherein the zoneplate is arranged in an X-ray microscope, and wherein, in a region nearthe optical axis of the X-ray microscope, zones of the zone plate arealigned parallel to said axis, and are increasingly inclined withrespect to the optical axis toward an edge of the zone plate.
 18. A zoneplate as claimed in claim 11, wherein the zone plate is arranged in anX-ray microscope, and wherein the zone plate is annularly constructedfor a condenser-monochromator, and a monochromator pinhole diaphragm isarranged at its focus.
 19. A zone plate as claimed in claim 11, whereinthe zone plate is an annular zone plate arranged in an X-ray microscope,and wherein a focussing device with a focussing ring is providedupstream in a beam path of the X-ray microscope to provide acondenser-monochromator, and wherein a monochromator pinhole diaphragmis arranged at the focus of the zone plate.
 20. A zone plate as claimedin claim 11, wherein the zone plate is arranged in an X-ray microscopeas a microscope objective.
 21. A zone plate as claimed in claim 11,wherein the zone plate has a rectilinear grating structure.